With a sharp increase of communication traffic, a 3rd Generation Partnership Project (3GPP) licensed frequency spectrum is more and more insufficient to provide higher network capacity. In order to further improve utilization of frequency spectrum resources, the 3GPP is discussing how to use unlicensed frequency spectrums, such as 2.4 GHz frequency bands and 5.8 GHz frequency bands, with help of licensed frequency spectrums. The unlicensed frequency spectrums are currently mainly used in systems such as Wi-Fi, BLUETOOTH™, radar, medical care, etc. Generally, an access technology designed for the licensed frequency band, such as a Long Term Evolution (LTE) is not suitable for using in the unlicensed frequency band, due to a fact that such access technologies like the LTE require quite high spectrum efficiency and user experience optimization requirement. However, a carrier aggregation function makes it be possible to deploy the LTE in the unlicensed frequency band. The 3GPP proposes a concept of Licensed-Assisted Access (LAA), the unlicensed frequency spectrum is used with the help of a LTSENDABLEE licensed frequency spectrum.
Interference level is guaranteed due to good orthogonality in a LTE network, then an uplink and downlink transmission between a base station and a user does not need to consider whether there is a base station around or a user is transmitting. When the LTE is used in the unlicensed frequency band, it is also not considered whether other devices are used in the unlicensed frequency band, then great interference is brought to a Wi-Fi device. As long as the LTE has business, it transmits without any monitoring rules, then the Wi-Fi device cannot transmit when the LTE has a service transmission. Only when a LTE service transmission is completed, it can detect an idle state of a channel and perform transmission.
Therefore, when the unlicensed frequency band is used by the LTE, one of main keys is to ensure that the LAA can coexist with existing access technologies (such as Wi-Fi) on a fair and friendly basis. A traditional LTE system does not have a mechanism of Listen Before Talk (LBT) to avoid collisions. In order to better coexist with Wi-Fi, the LTE requires an LBT mechanism. Then, when the channel is detected to be busy on an unlicensed carrier by the LTE, the frequency band cannot be occupied, and when the channel is detected to be idle, the frequency band can be occupied. A prior art does not provide a solution for sending a preamble on the unlicensed carrier, therefore how to perform random access on an uplink transmission preamble on the unlicensed carrier is a currently researched hotspot.